In application of touch detection on a touch screen or fingerprint identification, generally a touch operation or a fingerprint is detected by detecting capacitance variation. In a capacitance variation detection circuit, a circuit structure in the prior art is as illustrated in FIG. 1 and FIG. 2. In the state as illustrated in FIG. 1, switches S1, S3 and S5 are turned on and a detection capacitor Ctp is charged, charges stored in capacitors C1 and C2 are released, and other switches are all turned off. In the state as illustrated in FIG. 2, switches S2 and S4 are turned on, the other switches are turned off, charges stored in the detection capacitor Ctp are re-allocated, and according to the law of conservation of charge, the following equation is obtained:Q=CtpVpwr=CtpVref+C1Vref+C2(Vref−Vout)
An output after N times of sampling is:
      V    out    =            V      ref        -          N      ⁢                                                  C              tp                        ⁡                          (                                                V                  pwr                                -                                  V                  ref                                            )                                -                                    C              1                        ⁢                          V              ref                                                C          2                    
When the capacitance of the detection capacitor Ctp varies due to a touch press, a fingerprint or the like, the variation of the detection capacitor Ctp may be detected according to the output of the circuit. Ctp includes a basic value before variation, and a variation, which are respectively Cbg and Csig. After N times of sampling, the following equation is obtained:
      V    out    =            V      ref        -          N      ⁢                                                  C              bg                        ⁡                          (                                                V                  pwr                                -                                  V                  ref                                            )                                -                                    C              1                        ⁢                          V              ref                                                C          2                      -          N      ⁢                                    C            sig                    ⁡                      (                                          V                pwr                            -                              V                ref                                      )                                    C          2                    
It is apparent that since C1 is added, a useless charge variation caused by capacitance basic value Cbg is offset, and the variation portion may be only analyzed with respect to an output result.
However, during practice of the present disclosure, the inventors have identified that the above process has a problem:
Since Vpwr includes noise which comes from a driving circuit generating Vpwr, and other interference which is originated from a mobile or wearable device, including but not limited to interference from a screen driving signal or interference from a charger or interference from other parts of the chip. Therefore, after N times of sampling and integration, the noise and interference may also be increased with the integration. As a result, the output signal has an excessively low signal-to-noise ratio, which reduces the accuracy of touch detection or fingerprint identification.